1. Field of the Invention
This invention relates to an improved process for producing a tanned leather suitable for use for shoe soles, belts and straps, and bags and cases.
2. Description of the Prior Art
It is already known that various hides and skins can be tanned by the application of a variety of tanning agents including vegetable tanning compositions, mineral tanning agents such as chrome and zirconium tanning compounds, and synthetic tanning compositions (syntans). While vegetable tanning compositions are satisfactory for producing a variety of tanned leather products, such as sole leather from steer hides, upholstery and garment leather from cow hides, and glove leather from calf skin and the like, there is yet a need in the art for improved leather products, and methods for making the same, having improved abrasion resistance, diminished amount of water-extractable substances, improved strength properties, greater ease of fabrication, improved chemical resistance, elimination of the use of toxic tanning substances such as natural tanning extracts and phenolic tanning agents, and shorter tanning time.
Zirconium salts such as those disclosed in U.S. Pat. No. 2,826,477 to Rau and Somerville have been used to overcome some of the disadvantages previously known to be associated with the use of zirconium tanning agents.
Acrylic tanning agents, while suitable for many purposes, have the disadvantage of imparting brittleness and crackiness to the grain of the leather and of giving variable penetration of the tanning agent into the hide tanned therewith. U.S. Pat. No. 3,408,319 to Rau discloses the improvement in the use of synthetic acrylic polymeric tanning compositions containing units from acrylic acid and methacrylic acid and mixtures thereof wherein the disadvantages of imparting brittleness and crackiness to the grain of the leather are overcome by the use of a tanning composition made by copolymerizing acrylic acid, methacrylic acid, or mixtures thereof with a sulfated unsaturated drying oil. At column 3, lines 21-22 of the patent there appears the broad disclosure, referring to the tannage using the compounds of the patent, "This tannage may be followed by a mineral tanning, vegetable tanning, or synthetic tanning agent." Illustrative procedures 2, 4, 6 and 10 which appear in columns 3-5 of the patent exemplify the tannage of pickled calfskin to obtain white leather product using a synthetic copolymeric tanning composition disclosed in the patent as the single tanning agent. Illustrative procedure 8 exemplifies the pretannage of pickled calfskin with a synthetic copolymeric tanning composition of the patent followed by a tannage with basic zirconium sulfate. However, it is known in the leather tanning art that relatively thin calfskin tanned as set forth in illustrative procedure 8 of the patent, while suitable for use in making leathers requiring relatively narrow thickness such as lightweight dress shoe upper leather, would not be suitable for making leather articles requiring relatively broad thickness from thicker hides, such as sole leather from steer hides.
V. S. Shapirov et al., Kozh.-Obuvn. Prom-st., 20(3), 29-30 (1978); Chem. Abs., 88:171813p (1978), broadly disclose the tanning of shoe sole leathers using titanium-zirconium tanning compounds wherein the resulting products, optionally, may be further treated with syntans.
K. M. Zurabyan et al., USSR Pat. No. 561,733; Chem. Abs., 87:103388h (1977), disclose a process for beamhouse treatment and tanning of hide shoulders wherein the tanning involves the initial use of a chrome tanning composition followed by the use of an organic tanning agent. A rough translation of the Russian indicates that a vegetable tanning agent having phenolic components is used as the organic tanning agent.
It is conventional in the art of tanning leather to produce leather suitable for use in making shoe soles, belts and straps, and bags and cases by tanning hides with vegetable tanning compositions and, optionally, using mineral tanning agents and/or syntans in the final stages of the tanning process.
It is an object of the present invention to provide an improved process for tanning heavy leather, sometimes called full thickness leather, suitable for use for making shoe soles, belts and straps, bags and cases, and saddles, bridles and harness, whereby the leather product is characterized by an advantageous combination of, and an overall improvement in, properties not obtainable by processes heretofore known, such as a combination of improvements in appearance, fullness, firmness, flexibility, resilience, abrasion resistance, tensile strength, ease of fabrication and processing, water absorption, shrinkage temperature, content of water-extractable substances, chemical resistance, light-fastness, and density. It is another object to provide an improved tanned leather product produced by the process of the invention. It is yet another object to provide, as an article of manufacture, a shoe sole, belt, strap, bag, or case made from the tanned leather product produced by the process of the invention.